TUCSON, Ariz. -- On Mother's Day in 1994, a truck caught fire on a roadside in Saguaro National Park east of Tucson. Flames from the fire jumped into desert grasses and took off, burning 1,200 acres.

A few weeks later, biologists surveyed the charred area and found the carcasses of five burned tortoises. The fire killed an estimated 12 percent of the tortoises in the burn area -- a significant loss for a long-lived species that takes a decade or more to reach sexual maturity.While the fire's direct effects on tortoises were dramatic and obvious, its lingering aftereffects are subtle and more difficult to measure. But fires such as the one on that Mother's Day are slowly transforming the Sonoran Desert in ways that may make it more difficult for tortoises to survive here, said researcher Todd C. Esque of the U.S. Geological Survey.

With funding from the National Park Service, a team of scientists led by Esque is investigating the long-term effects of fires on tortoises and another long-lived Sonoran Desert resident, the saguaro.

"The data we're providing on tortoises and saguaros indicates we actually are having significant population changes in the vegetation and tortoise populations," he said.

In 1994, 7.2 percent of the saguaros in the burned zone died. The following year, saguaro mortality jumped to 16.2 percent, then dropped to 2.4 percent a year later. In the unburned study area, annual saguaro mortality has remained around 2 percent to 3 percent.

Wildfire in the Sonoran Desert is an emerging problem: Until recently, desert vegetation was too sparse to support big fires. But the insidious spread of nonnative grasses -- especially red brome and buffel grass -- has carried the fire threat into a desert ecosystem that evolved without flames and cannot tolerate them.

Red brome was partly to blame for the Mother's Day Fire, Esque said.

Wildfires kill saguaros, palo verdes and other desert plants, and desert tortoises can be cooked in their shells unless they're inside burrows or caves when the flames sweep through.

But exotic grasses thrive on fires and exploit them to grab more territory. While native plants perish, the grasses simply re-sprout from the roots and creep farther into the desert, growing more and more dense.

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The result of this grass-fire cycle, after repeated intense burns over many years, is a less diverse landscape dominated by exotic grasses, Esque said.

Fewer trees, shrubs and cactuses in the desert means less shade for desert tortoises.

A desert tortoise will die if its core body temperature rises above 104 degrees, Esque said. Biologists suspect that tortoises avoid overheating and maximize the time they can spend feeding by "shuttling" back and forth between sunlight and shade.

Lack of shade makes it more difficult for tortoises to get the food they need, Esque said.

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